This invention relates to an identification method and an identification apparatus for identifying an unknown system by the use of an adaptive filter. It is to be noted here that such an adaptive filter is used as an echo canceller, a noise canceller, a howling canceller, an adaptive equalizer, and the like to identify the unknown system, such as a transmission line and a spatial acoustic coupling path, although the following description will be mainly directed to an acoustic echo canceller for cancelling an acoustic echo leaking from a loudspeaker to a microphone in a spatial acoustic coupling path.
The echo canceller is operable so that the acoustic echo leaking from the loudspeaker to the microphone in the spatial acoustic coupling path is canceled by generating a pseudo echo (echo replica) corresponding to a transmission signal by the use of the adaptive filter which has taps of a number covering a time interval longer than the duration of an impulse respons of an echo path. A tap coefficient of each tap of the adaptive filter is modified by monitoring a correlation between a far-end signal and an error signal obtained by subtracting the echo replica from a mixture signal comprising a mixture of the echo and a near-end signal. As typical algorithms for modifying the tap coefficients of the adaptive filter, an LMS (least mean square) algorithm and a learning identification method (LIM) are disclosed in articles which are contributed by B. Widrow et al to Proceedings of IEEE, Vol. 63, No. 12, December, 1975, pages 1692-1716 (will hereinunder be called "Document 1") and contributed by J. Nagumo et al to IEEE Transactions on Automatic Control, Vol. AC-12, No. 3, 1967, pages 282-287 (will hereinunder be called "Document 2"), respectively.
An acoustic space in which the acoustic echo canceller is practically used produces the impulse response having a duration dependent upon a physical dimension of the acoustic space and a reflectivity of a wall or the like. When the acoustic space is difined by a room used in a video conference, the duration of the impulse response corresponds to 1000 taps and sometimes corresponds to several thousands of taps. In view of the calculation amount/the hardware scale, it is often difficult to implement. In order to mainly remove the disadvantage of an increase in calculation amount, adaptive filters of a band-division type (namely, a subband type) are overviewed in an article which is contributed by J. J. Shynk to IEEE SP Magazine, January, 1992, pages 14-37 (will hereinunder be called "Document 3"). The adaptive filter may have various structures for each subband. As a most popular adaptive filter, a FIR (finite impulse response) type adaptive filter is used as described in an article which is contributed by N. A. M. Verhoeck et al to IEEE Transactions on Acoustics, Speech, and Signal Processing, Vol. ASSP-27, No. 6, December, 1979, pages 768-781 (will hereunder be called "Document 4").
The number of taps N of the FIR type adaptive filter corresponding to each subband must be equal to or longer than the duration of the corresponding impulse response in time. In general, when the acoustic echo is subjected to band division, a low-band impulse response is longer than a high-band impulse response as will later be illustrated. This is because the impulse response of the acoustic echo has a duration determined mainly by reflection and because the high-band component has a smaller reflection coefficient and is readily attenuated.
Taking these characteristics into consideration, proposal is made of a band-division adaptive filter which is capable of reducing the calculation amount and shortening a convergence time by adaptively controlling the number of taps of the adaptive filter for each subband. The band-division adaptive filter is disclosed in an article which is contributed by Akihiko Sugiyama to Proceedings of 1994 Spring Conference of Institute of Electronics, Information and Communication Engineers, March 1994, page 1-173 (will hereunder be called "Document 5"). The above-named Akihiko Sugiyama is the instant applicant.
As will later be described, a conventional identification method and apparatus using the band-division adaptive filter disclosed in the Document 5 is incapable of quickly identifying the unknown system with a final error reduced.